Electric lighting circuits of the type used in theater rely heavily on dimmer circuits which present a nonlinear electrical load to the AC line. Typically dimmers use thyristors for controlling the switching of current into a lighting load. A typical thyristor-controlled lighting load is shown in Pearlman et al. U.S. Pat. No. 4,649,323. Using a technique called forward phase control, a firing signal is sent to a thyristor at some predetermined phase angle after the zero crossing of the AC current which turns on the current to the load. The phase control of this type, when using thyristors and other switched loads, presents problems for reactive components such as motor drives coupled to the load and associated circuitry. One major effect of nonlinearities in the load is the creation of harmonic currents which have frequencies that are multiples of the line frequency. Harmonic currents which are odd multiples of the AC line frequency may cause excessive heating in the magnetic steel cores of transformers and neutral wires. These especially troublesome odd order harmonics tend to be additive in the neutral conductors of the system. This overloads the neutral conductors and causes excessive heating.
In an ideal three phase, four wire system with no harmonic currents, single phase line-to-neutral load currents flow in each phase conductor and return in the common neutral conductor. This would represent the base line neutral current. If the three 60 cycle phase currents are separated by 120.degree. for a balanced three phase load the currents should be equal. When the return currents flow in the neutral wire, there should be cancellation, making the net current zero at all points.
In a system with phase controlled loads, however, the return currents include these third harmonic currents. The sum of the total of the three phases at the third harmonic is, however, the arithmetic sum of the individual third harmonic phase currents. This is also true for other odd multiples of the third harmonic. The theoretical maximum neutral current with harmonics is at least 1.73 and perhaps as much as three times the phase current. This is dependent to a degree on the type of load but for phase control lighting loads it is about 1.37 times the phase current. As pointed out above, this unwanted current in the neutral wire can cause over-heating and can effect the power factor. The effect on the service transformer is that load currents which are substantially high in harmonic content can cause more heating than an undistorted current. This is due to the fact that heating is related to the square of the frequency of the current. Thus, a third order harmonic will have nine times the heating effect of the line current. Any current losses also increase as a function of frequency. In such situations transformers which have been loaded to only 70% of their rated load have been shut down due to over-temperature conditions. Also, the service transformer for an entertainment lighting system is subjected to enormous stresses due to cold lamp in-rush currents which may be 15 to 20 times the normal rated lamp current. In-rush currents coupled with third order harmonics can drastically reduce the service life of these transformers.
What is needed, therefore, is a method for reducing the level of odd harmonic currents in the neutral line to reduce heating and transformer stress.
It has been discovered that odd order harmonics produced by a dimmer operating in forward phase control are out of phase with the odd order harmonics produced by a dimmer operating in reverse phase control. Recently, circuits have been developed for dimmers of the stage lighting type that operate in reverse phase control. Dimmer control circuits of this type are shown in Schanin et al. U.S. Pat. Nos. 5,239,255 and 5,004,969, the disclosures of which are hereby incorporated herein by reference. In addition, the Schanin et al. U.S. Pat. No. 5,239,255 includes the capability for operating in either reverse phase control (RPC) mode or forward phase control (FPC) mode.